Conventionally, a brake device such as a rail brake has been used to prevent runaway of a quay crane or portal crane during normal loading or under a weather where a sudden gust of wind possibly occurs. A pressing-type brake device (see, for example, Patent Literature 1) and a pinching-type brake device (see, for example, Patent Literature 2) are mainly disclosed as the conventional brake devices.
First, the pressing-type brake device described in Patent Literature 1 will be described. The pressing-type brake device includes plural disc springs stacked with each other, a brake shoe fixed at the lower end portion of the stacked disc springs, and a hydraulic cylinder for compressing the disc springs.
This brake device is configured such that the disc springs are compressed by the hydraulic cylinder so that the brake shoe is not brought into contact with the rail during a normal time, for example, when the crane travels. Furthermore, when the brake device is activated, the disc springs are released, and the brake shoe is pressed against the rail with the restoring force of the disc springs. With this configuration, the brake shoe generates frictional force with the upper surface of the rail, whereby runaway of the crane can be prevented.
However, the pressing-type brake device described above has several problems. The first problem is that the manufacturing cost of the brake device significantly increases to improve the braking force of the brake device. This is because the sizes of the disc springs and the hydraulic cylinder need to be increased to improve the braking force, which leads to an increase in the manufacturing cost.
The second problem is that it is difficult to reduce the size of the brake device. This is because the number of disc springs each having a small deformation amount needs to be increased to ensure the stroke of the brake shoe with respect to the rail. More specifically, the rail warps in the vertical direction by approximately ±10 to 30 mm. For this reason, the brake shoe needs to have a stroke of approximately 40 mm. Here, because the deformation amount per disc spring is approximately 1 to 2 mm, at least 20 to 40 disc springs need to be stacked, which leads to an increase in the size of the brake device.
The third problem is that it is difficult to improve reliability of the brake device. This is because disc springs may crack, which leads to a significant reduction in the braking force. The brake device is repeatedly activated and stopped every time the crane travels and stops, and is frequently used, which increases the possibility of breakage of built-in disc springs.
Next, the pinching-type brake device described in Patent Literature 2 will be described. The pinching-type brake device includes brake shoes configured to pinch the rail from both sides thereof, and a motor having a cam for pressing the brake shoes against the rail. With this configuration, the brake shoes generate frictional force with the side surfaces of the rail, whereby runaway of the crane can be prevented.
However, the pinching-type brake device has several problems. The first problem is that the manufacturing cost of the brake device significantly increases to improve the braking force of the brake device. This is because the size of the motor needs to be increased to improve the braking force, which leads to an increase in the cost of the motor.
The second problem is that there is a possibility that brake does not work in the case where the brake device is activated during runaway of the crane. This is because the main body of the pinching-type brake device may be flicked upward and the rail may not be held as a result of operations of upper and lower position adjusting mechanisms, which are installed in the main body of the brake device.